Failure characteristics of two porous sandstones subjected to true triaxial stresses

Abstract

AbstractWe conducted an extensive suite of true triaxial experiments in two porous sandstones, Bentheim (porosity ≈ 24%) and Coconino (17.5%). Our experiments demonstrate that failure of both sandstones is not only a function of σ3 but also of σ2. For a given σ3, σ1 at failure (σ1,peak) increases as σ2 is raised above σ3 between tests. The σ1,peak reaches a peak as σ2 is about halfway between σ3 and σ1 and then gradually decreases such that when σ2 ≈ σ1,peak, it approaches its initial magnitude when σ2 = σ3. For a constant σ3, failure‐plane angle increases with σ2 by a maximum of less than 10° as σ2 rises from σ2 = σ3 to σ2 = σ1,peak. The effect of σ2 on both failure level and failure‐plane angle is stronger in the lower‐porosity Coconino sandstone than in the Bentheim sandstone. The σ2 dependence of failure mode in the Bentheim is different than Coconino over the same σ3 range. Both sandstones failed dilatantly at low σ3 magnitudes. However, at high σ3 (100–120 MPa), Bentheim sandstone developed shear‐enhanced compaction bands, followed by pure compaction bands at σ3 = 150 MPa. Compaction bands were not observed in the Coconino. Microscopic observations via SEM reveal that tensile microcracking is dominant when shear banding occurs (under low σ3), while pervasive grain crushing and pore collapse inside compaction bands are observed at high σ3.